Wet state control device for fuel cell

ABSTRACT

A wet state control device for fuel cell includes a priority control unit for preferentially controlling either one of a pressure and a flow rate of cathode gas when a wet state of a fuel cell is adjusted, a water temperature control unit for controlling a temperature of cooling water when the wet state of the fuel cell is not completely adjustable by a control of the priority control unit, and a complementary control unit for controlling the other of the pressure and the flow rate of the cathode gas to complement a response delay of the water temperature control unit.

TECHNICAL FIELD

This invention relates to a device for controlling a wet state of a fuelcell.

BACKGROUND ART

For efficient power generation of a fuel cell, it is important tomaintain an electrolyte membrane in a suitable wet state. Specifically,if the electrolyte membrane is too wet, flooding occurs or a purgeoperation at stoppage is necessary in preparation for sub-zero start-up.Further, if the electrolyte membrane is insufficiently wet, a voltage ofa fuel cell stack may drop and an output may be largely reduced.Accordingly, in JP2007-115488A issued by the Japan Patent Office in2007, a pressure regulating valve and a cathode compressor arecontrolled to set such cathode gas pressure and cathode gas flow ratethat an electrolyte membrane is maintained in a suitable wet state.Particularly in the case of controlling to make the electrolyte membranewetter in consideration of fuel economy, a rotation speed is firstreduced to lower power consumption of the cathode compressor and thenthe pressure regulating valve is closed to increase the pressure.

SUMMARY OF INVENTION

Cooling water temperature is one of parameters for controlling a degreeof wetness. However, the cooling water temperature is not controlled toregulate wetness in the aforementioned technique. Thus, the presentinventors found that there was room for improvement of fuel economy in awetness control in transition.

The present invention was developed in view of such a conventionalproblem. The present invention aims to provide a wet state controldevice for fuel cell capable of maintaining an electrolyte membrane in asuitable wet state while suppressing the deterioration of fuel economyincluding a cooling water control.

A wet state control device for fuel cell according to a certainembodiment of the present invention includes a priority control unit forpreferentially controlling either one of a pressure and a flow rate ofcathode gas when a wet state of a fuel cell is adjusted, a watertemperature control unit for controlling a temperature of cooling waterwhen the wet state of the fuel cell is not completely adjustable by acontrol of the priority control unit, and a complementary control unitfor controlling the other of the pressure and the flow rate of thecathode gas to complement a response delay of the water temperaturecontrol unit.

Embodiments of the present invention and advantages thereof aredescribed in detail below in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing an example of a system to which a wet statecontrol device for fuel cell according to the present invention isapplied,

FIG. 2A is a schematic diagram showing a reaction of an electrolytemembrane in a fuel cell stack,

FIG. 2B is a schematic diagram showing the reaction of the electrolytemembrane in the fuel cell stack,

FIG. 3 is a block diagram showing functions relating to a wet statecontrol of a controller when a target wet state decreases,

FIG. 4 are timing charts showing the operation of the wet state controldevice when the target wet state decreases,

FIG. 5 are charts showing a problem during execution of the abovecontrol logic when the target wet state increases,

FIG. 6 is a block diagram showing functions relating to the wet statecontrol of the controller when the target wet state increases,

FIG. 7 is a graph showing temperature input to a target pressurecalculation block B101 of the wet state control device for fuel cellaccording to the present invention,

FIG. 8 are timing charts showing the operation of the wet state controldevice when the target wet state increases,

FIG. 9A is a block diagram showing functions relating to a wet statecontrol of a controller of a second embodiment of a wet state controldevice for fuel cell according to the present invention,

FIG. 9B is a block diagram showing functions relating to the wet statecontrol of the controller of the second embodiment of the wet statecontrol device for fuel cell according to the present invention,

FIG. 10 are timing charts showing the operation of the wet state controldevice when a target wet state increases,

FIG. 11 are timing charts showing the operation of the wet state controldevice when the target wet state decreases,

FIG. 12A is a block diagram showing functions relating to a control of acontroller in a third embodiment of a wet state control device for fuelcell according to the present invention,

FIG. 12B is a block diagram showing functions relating to the control ofthe controller in the third embodiment of the wet state control devicefor fuel cell according to the present invention,

FIG. 13 are timing charts showing the operation of the wet state controldevice when a target wet state decreases,

FIG. 14 are timing charts showing the operation of the wet state controldevice when the target wet state increases,

FIG. 15A is a block diagram showing functions relating to a control of acontroller in a fourth embodiment of a wet state control device for fuelcell according to the present invention,

FIG. 15B is a block diagram showing functions relating to the control ofthe controller in the fourth embodiment of the wet state control devicefor fuel cell according to the present invention,

FIG. 16A is a block diagram showing functions relating to a control of acontroller in a fifth embodiment of a wet state control device for fuelcell according to the present invention,

FIG. 16B is a block diagram showing functions relating to the control ofthe controller in the fifth embodiment of the wet state control devicefor fuel cell according to the present invention,

FIG. 17A is a block diagram showing functions relating to a control of acontroller in a sixth embodiment of a wet state control device for fuelcell according to the present invention, and

FIG. 17B is a block diagram showing functions relating to the control ofthe controller in the sixth embodiment of the wet state control devicefor fuel cell according to the present invention.

DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 1 is a diagram showing an example of a system to which a wet statecontrol device for fuel cell according to the present invention isapplied.

First, a basic system to which the wet state control device for fuelcell according to the present invention is applied is described withreference to FIG. 1.

A fuel cell stack 10 generates power by being supplied with reaction gas(cathode gas O₂, anode gas H₂) while being maintained at a suitabletemperature. Accordingly, a cathode line 20, an anode line 30 and acooling water circulation line 40 are connected to the fuel cell stack10. It should be noted that a power generation current of the fuel cellstack 10 is detected by a current sensor 101. A power generation voltageof the fuel cell stack 10 is detected by a voltage sensor 102.

Cathode gas O₂ to be supplied to the fuel cell stack 10 flows in thecathode line 20. A compressor 21 and a cathode pressure regulating valve22 are provided in the cathode line 20.

The compressor 21 supplies the cathode gas O₂, i.e. air to the fuel cellstack 10. The compressor 21 is provided upstream of the fuel cell stack10 in the cathode line 20. The compressor 21 is driven by a motor M. Thecompressor 21 regulates a flow rate of the cathode gas O₂ flowing in thecathode line 20. The flow rate of the cathode gas O₂ is regulated by therotation speed of the compressor 21.

The cathode pressure regulating valve 22 is provided downstream of thefuel cell stack 10 in the cathode line 20. The cathode pressureregulating valve 22 regulates a pressure of the cathode gas O₂ flowingin the cathode line 20. The pressure of the cathode gas O₂ is regulatedby an opening of the cathode pressure regulating valve 22.

The flow rate of the cathode gas O₂ flowing in the cathode line 20 isdetected by a cathode flow rate sensor 201. This cathode flow ratesensor 201 is provided downstream of the compressor 21 and upstream ofthe fuel cell stack 10.

The pressure of the cathode gas O₂ flowing in the cathode line 20 isdetected by a cathode pressure sensor 202. This cathode pressure sensor202 is provided downstream of the compressor 21 and upstream of the fuelcell stack 10. Further, in FIG. 1, the cathode pressure sensor 202 islocated downstream of the cathode flow rate sensor 201.

The anode gas H₂ to be supplied to the fuel cell stack 10 flows in theanode line 30. An anode recirculation line 300 is arranged in parallelto the anode line 30. The anode recirculation line 300 is branched offfrom the anode line 30 at a side downstream of the fuel cell stack 10and joins the anode line 30 at a side upstream of the fuel cell stack10. A bomb 31, an anode pressure regulating valve 32, an ejector 33, ananode pump 34 and a purge valve 35 are provided in the anode line 30.

The anode gas H₂ is stored in a high pressure state in the bomb 31. Thebomb 31 is provided at a most upstream side of the anode line 30.

The anode pressure regulating valve 32 is provided downstream of thebomb 31. The anode pressure regulating valve 32 regulates a pressure ofthe anode gas H₂ to be newly supplied from the bomb 31 to the anode line30. The pressure of the anode gas H₂ is regulated by an opening of theanode pressure regulating valve 32.

The ejector 33 is provided downstream of the anode pressure regulatingvalve 32. The ejector 33 is located at a position where the anoderecirculation line 300 joins the anode line 30. The anode gas H₂ havingflowed in the anode recirculation line 300 is mixed with the anode gasH₂ newly supplied from the bomb 31 by this ejector 33.

The anode pump 34 is located downstream of the ejector 33. The anodepump 34 feeds the anode gas H₂ having flowed through the ejector 33 tothe fuel cell stack 10.

The purge valve 35 is provided downstream of the fuel cell stack 10 in aside of the anode line 30 downstream of a position where the anoderecirculation line 300 is branched off. When the purge valve 35 isopened, the anode gas H₂ is purged.

A pressure of the anode gas H₂ flowing in the anode line 30 is detectedby an anode pressure sensor 301. This anode pressure sensor 301 isprovided downstream of the anode pump 34 and upstream of the fuel cellstack 10.

Cooling water to be supplied to the fuel cell stack 10 flows in thecooling water circulation line 40. A radiator 41, a three-way valve 42and a water pump 43 are provided in the cooling water circulation line40. Further, a bypass line 400 is arranged in parallel to the coolingwater circulation line 40. The bypass line 400 is branched off at a sideupstream of the radiator 41 and joins at a side downstream of theradiator 41. Thus, the cooling water flowing in the bypass line 400bypasses the radiator 41.

The radiator 41 cools the cooling water. The radiator 41 is providedwith a cooling fan 410.

The three-way valve 42 is located at a position where the bypass line400 joins the cooling water circulation line 40. The three-way valve 42regulates a flow rate of the cooling water flowing in a radiator sideline and that of the cooling water flowing in the bypass line. In thisway, the temperature of the cooling water is regulated.

The water pump 43 is located downstream of the three-way valve 42. Thewater pump 43 feeds the cooling water having flowed through thethree-way valve 42 to the fuel cell stack 10.

The temperature of the cooling water flowing in the cooling watercirculation line 40 is detected by a water temperature sensor 401. Thiswater temperature sensor 401 is provided upstream of a position wherethe bypass line 400 is branched off.

To a controller are input signals of the current sensor 101, thepressure sensor 102, the cathode flow rate sensor 201, the cathodepressure sensor 202, the anode pressure sensor 301 and the watertemperature sensor 401. Then, the controller outputs signals to controlthe operations of the compressor 21, the cathode pressure regulatingvalve 22, the anode pressure regulating valve 32, the anode pump 34, thepurge valve 35, the three-way valve 42 and the water pump 43.

By such a configuration, the fuel cell stack 10 generates power by beingsupplied with reaction gas (cathode gas O₂, anode gas H₂) while beingmaintained at a suitable temperature. The power generated by the fuelcell stack 10 is supplied to a battery 12 and a load 13 via a DC/DCconverter 11.

FIGS. 2A and 2B are schematic diagrams showing reactions of anelectrolyte membrane in the fuel cell stack.

Next, a technical idea of the inventors is described with reference toFIGS. 2A and 2B.

As described above, the fuel cell stack 10 generates power by beingsupplied with reaction gas (cathode gas O₂, anode gas H₂). The fuel cellstack 10 is constructed by stacking several hundreds of membraneelectrode assemblies (MEAs) each formed with a cathode electrodecatalyst layer and an anode electrode catalyst layer on oppositesurfaces of the electrolyte membrane. It should be noted that FIG. 2Ashows one MEA. Here is shown an example in which cathode gas is supplied(cathode-in) and discharged from a diagonal side (cathode-out) whileanode gas is supplied (anode-in) and discharged from a diagonal side(anode-out).

In each membrane electrode assembly (MEA), the following ions proceedaccording to a load in the cathode electrode catalyst layer and theanode electrode catalyst layer to generate power.[Equations 1]Cathode electrode catalyst layer: 4H⁺+4e ⁻+O₂→2H₂O  (1-1)Anode electrode catalyst layer: 2H₂→4H⁺+4e ⁻  (1-2)

As shown in FIG. 2B, the reaction of the above Equation (1-1) proceedsand water vapor is produced as the reaction gas (cathode gas O₂) flowsin a cathode flow passage. Then, relative humidity increases at adownstream side of the cathode flow passage. As a result, a relativehumidity difference between a cathode side and an anode side becomeslarger. Water reversely diffuses to humidify an upstream side of theanode using this relative humidity difference as a driving force. Thismoisture further evaporates from the MEA into the anode flow passage andhumidifies the reaction gas (anode gas H₂) flowing in the anode flowpassage. Then, the moisture is carried to a downstream side of the anodeto humidify the MEA downstream of the anode.

To efficiently generate power by the above reactions, it is necessaryfor the electrolyte membrane to be in a suitable wet state.

Accordingly, the present inventors focused attention on the flow rateand the pressure of the cathode gas O₂ and the temperature of the fuelcell stack 10.

Specifically, if the flow rate of the cathode gas O₂ is increased, themoisture discharged together with the cathode gas O₂ increases.Accordingly, the wet state of the electrolyte membrane can be decreased.On the other hand, if the flow rate of the cathode gas O₂ is decreased,the moisture discharged together with the cathode gas O₂ decreases.Accordingly, the wet state of the electrolyte membrane can be increased.

The pressure of the cathode gas O₂ decreases when the opening of thecathode pressure regulating valve 22 increases. Accordingly, if thepressure of the cathode gas O₂ is decreased by increasing the opening ofthe cathode pressure regulating valve 22, the cathode gas O₂ is moreeasily discharged. As a result, the moisture discharged together withthe cathode gas O₂ also increases. Thus, the wet state of theelectrolyte membrane can be decreased. On the other hand, the pressureof the cathode gas O₂ increases when the opening of the cathode pressureregulating valve 22 decreases. Accordingly, if the pressure of thecathode gas O₂ is increased by decreasing the opening of the cathodepressure regulating valve 22, the cathode gas O₂ becomes hard todischarge. As a result, the moisture discharged together with thecathode gas O₂ also decreases. Thus, the wet state of the electrolytemembrane can be increased.

If the temperature of the fuel cell stack 10 increases, the moistureincluded in the cathode gas O₂ increases. As a result, the moisturedischarged together with the cathode gas O₂ also increases. Thus, thewet state of the electrolyte membrane can be decreased. On the otherhand, if the temperature of the fuel cell stack 10 decreases, themoisture included in the cathode gas O₂ decreases. As a result, themoisture discharged together with the cathode gas O₂ also decreases.Thus, the wet state of the electrolyte membrane can be increased.

The inventors obtained such knowledge. Further, if the rotation speed ofthe compressor 21 is increased to increase the flow rate of the cathodegas O₂, power consumption increases to deteriorate fuel economy.Accordingly, it is desirable to suppress the rotation speed of thecompressor 21 as low as possible. The inventors have completed thepresent invention based on such an idea. Specific contents are describedbelow.

FIG. 3 is a block diagram showing functions relating to a wet statecontrol of the controller when a target wet state decreases.

It should be noted that each block shown in the block diagram shows eachfunction of the controller as a virtual unit and does not mean physicalexistence.

The wet state control device controls the wet state of the electrolytemembrane of the fuel cell stack 10 by controlling the operations of thecompressor 21, the cathode pressure regulating valve 22, the anodepressure regulating valve 32, the anode pump 34, the purge valve 35, thethree-way valve 42 and the water pump 43. Specifically, the wet statecontrol device includes a target pressure calculation block B101, atarget temperature calculation block B102 and a target flow ratecalculation block B103. It should be noted that, in the presentembodiment, the target pressure calculation block B101 corresponds to apriority control unit as claimed, the target temperature calculationblock B102 corresponds to a water temperature control unit as claimedand the target flow rate calculation block B103 corresponds to acomplementary control unit as claimed.

The target pressure calculation block B101 calculates a target pressureP_(target) based on a target water discharge quantity Q_(H2O) _(_)_(out), a minimum stack temperature T_(min) and a minimum cathode flowrate Q_(min) when the target water discharge quantity Q_(H2O) _(_)_(out) increases, i.e. at the time of drying by decreasing the wetstate.

It should be noted that the target water discharge quantity Q_(H2O) _(_)_(out) [NL/min] is calculated by the following Equation (2). Here NLdenotes normal liter, i.e. liter in a normal state.[Equation 2]Q _(H2O) _(_) _(out) =Q _(H2O) _(—in) −Q _(net) _(_) _(water)  (2)where:

Q_(H2O) _(_) _(out): target water discharge quantity [NL/min]

Q_(H2O) _(_) _(in): quantity of water produced in fuel cell [NL/min]

Q_(net) _(_) _(water): target water balance [NL/min]

It should be noted that the quantity Q_(H2O) _(_) _(in) of waterproduced in fuel cell [NL/min] is calculated by the following Equation(3).

$\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack & \; \\{Q_{H\; 2{O\_ in}} = {\frac{NI}{2\; F} \times 22.4 \times 60}} & (3)\end{matrix}$where:

N: number of cells of fuel cell

I: output current [I] of fuel cell

F: Faraday constant (96485.39 [C/mol]

22.4: volume [NL] of 1 mol of ideal gas in normal state

60: unit conversion coefficient between second and minute

The target water balance Q_(net) _(_) _(water) [NL/min] is set torealize the target wet state of the electrolyte membrane determinedaccording to an operating state (load state) of the fuel cell.

The target pressure calculation block B101 calculates the targetpressure P_(target) based on the thus obtained target water dischargequantity Q_(H2O) _(_) _(out), the minimum stack temperature T_(min) andthe minimum cathode flow rate Q_(min). Specifically, the target pressureP_(target) is calculated by the following Equations (4-1), (4-2).

$\begin{matrix}\left\lbrack {{Equations}\mspace{14mu} 4} \right\rbrack & \; \\{{P_{target} = {P_{sat\_ min}\frac{Q_{\min} + Q_{H\; 2{O\_ out}}}{Q_{H\; 2{O\_ out}}}}}{where}} & \left( {4\text{-}1} \right) \\{P_{sat\_ min} = 10^{7.7406 - \frac{1657.46}{227.02 + T_{\min}}}} & \left( {4\text{-}2} \right)\end{matrix}$

Here, the minimum stack temperature T_(min) is a stack temperature atthe time of setting the fuel cell stack in a maximum wet state. Asdescribed above, the temperature of the fuel cell stack 10 is decreasedto increase the wet state of the electrolyte membrane. It should benoted that a power generation failure may be caused by condensed waterif the temperature of the fuel cell stack 10 is too low. On the otherhand, if this temperature is too high, the deterioration of the fuelcell stack 10 is accelerated. Thus, the stack temperature at the time ofsetting the fuel cell stack in the maximum wet state is a lowest stacktemperature in a range where the performance of the fuel cell stack canbe ensured, comprehensively considering these. Similarly, the minimumcathode flow rate Q_(min) is a cathode flow rate at the time of settingthe fuel cell stack in the maximum wet state. As described above, thecathode flow rate is decreased to increase the wet state of theelectrolyte membrane. It should be noted that a power generation failuremay be caused by an insufficient supply amount if the cathode flow rateis too low. On the other hand, if the cathode flow rate is too high,sound vibration performance may be decelerated. Thus, the cathode flowrate at the time of setting the fuel cell stack in a maximum wet stateis a lowest cathode flow rate in a range where the performance of thefuel cell stack can be ensured, comprehensively considering these. Theseare set according to the operating state of the fuel cell by anexperiment in advance.

Further, P_(sat) _(_) _(min) denotes a saturated water vapor pressure atthe minimum stack temperature T_(min) and the above Equation (4-2) isobtained based on Antoine equation.

In the above manner, the target pressure calculation block B101calculates the target pressure P_(target) based on the target waterdischarge quantity Q_(H2O) _(_) _(out), the minimum stack temperatureT_(min) and the minimum cathode flow rate Q_(min) when the target waterdischarge quantity Q_(H2O) _(_) _(out) increases, i.e. at the time ofdrying by decreasing the wet state.

The target temperature calculation block B102 calculates a targettemperature T_(target) based on the target water discharge quantityQ_(H2O) _(_) _(out), a pressure P_(sens) detected by the cathodepressure sensor 202 and the minimum cathode flow rate Q_(min).Specifically, the target temperature T_(target) is calculated by thefollowing Equations (5-1), (5-2). It should be noted that Equation (5-1)is obtained by the reverse of Antoine equation.

$\begin{matrix}\left\lbrack {{Equations}\mspace{14mu} 5} \right\rbrack & \; \\{{T_{target} = {\frac{1657.46}{7.7406 - {\log_{10}P_{sat\_ target}}} - 227.02}}{{where}\text{:}}} & \left( {5\text{-}1} \right) \\{P_{sat\_ target} = {P_{sens}\frac{Q_{H\; 2{O\_ out}}}{Q_{\min} + Q_{H\; 2{O\_ out}}}}} & \left( {5\text{-}2} \right)\end{matrix}$

P_(sat) _(_) _(target) is a target saturated water vapor pressure. Itshould be noted that although the pressure P_(sens) is detected by thecathode pressure sensor 202, a pressure loss of the fuel cell stack maybe obtained by an experiment in advance and the pressure P_(sens) may beestimated based on that.

In the above manner, the target temperature calculation block B102calculates the target temperature T_(target) based on the target waterdischarge quantity Q_(H2O) _(_) _(out), the actual pressure P_(sens) andthe minimum cathode flow rate Q_(min).

The target flow rate calculation block B103 calculates a target cathodeflow rate Q_(target) based on the target water discharge quantityQ_(H2O) _(_) _(out), the pressure P_(sens) detected by the cathodepressure sensor 202 and a water temperature T_(sens) detected by thewater temperature sensor 401. Specifically, the target cathode flow rateQ_(target) is calculated by the following Equations (6-1), (6-2).

$\begin{matrix}\left\lbrack {{Equations}\mspace{14mu} 6} \right\rbrack & \; \\{{Q_{target} = {Q_{H\; 2{O\_ out}}\frac{P_{sens} - P_{sat\_ sens}}{P_{sat\_ sens}}}}{{where}\text{:}}} & \left( {6\text{-}1} \right) \\{P_{sat\_ sens} = 10^{7.7406 - \frac{1657.46}{227.02 + T_{sens}}}} & \left( {6\text{-}2} \right)\end{matrix}$

P_(sat) _(_) _(sens) is a saturated water vapor pressure at the watertemperature T_(sens) detected by the water temperature sensor 401.

In the above manner, the target flow rate calculation block B103calculates the target cathode flow rate Q_(target) based on the targetwater discharge quantity Q_(H2O) _(_) _(out), the actual pressureP_(sens) and the actual water temperature T_(sens).

FIG. 4 are timing charts showing the operation of the wet state controldevice when the target wet state decreases.

When the above control logic is executed, the wet state control deviceoperates as follows when the target wet state decreases.

When the target wet state decreases at time t11, the wet state controldevice starts operating.

The target pressure P_(target) is set based on the target waterdischarge quantity Q_(H2O) _(_) _(out), the minimum stack temperatureT_(min) and the minimum cathode flow rate Q_(min). The targettemperature T_(target) is set based on the target water dischargequantity Q_(H2O) _(_) _(out), the actual pressure P_(sens) and theminimum cathode flow rate Q_(min). The target cathode flow rateQ_(target) is set based on the target water discharge quantity Q_(H2O)_(_) _(out), the actual pressure P_(sens) and the actual watertemperature T_(sens).

Since being set based on the stack temperature (minimum stacktemperature T_(min) and the cathode flow rate (minimum cathode flow rateQ_(min)) at the time of setting the maximum wet state, the targetpressure P_(target) is most likely to vary. Accordingly, the targetpressure P_(target) is first preferentially decreased. Then, the cathodepressure regulating valve 22 is controlled to realize this targetpressure P_(target). Then, the cathode pressure decreases almost withoutany response delay.

If a complete adjustment is not possible only by changing the targetpressure P_(target), the target temperature T_(target) starts varying attime t12. Specifically, a limit value (minimum cathode flow rateQ_(min)) is used to set the target temperature T_(target). Further, thesensor detection value P_(sens) of the cathode pressure regulated asdescribed above is fed back. Thus, an amount unadjustable only by thecathode pressure is adjusted by changing the temperature of the coolingwater. It should be noted that the temperature of the cooling water isunlikely to vary and a response delay is likely to occur even if atarget value is changed. Since the temperature of the cooling water isdetected by the water temperature sensor 401 and fed back to determinethe cathode flow rate, a response delay of the cooling water temperatureis complemented by the cathode flow rate.

If a complete adjustment is not possible even if the target temperatureT_(target) is changed, the target cathode flow rate Q_(target) startsvarying at time t13. Specifically, since the pressure P_(sens) detectedby the cathode pressure sensor 202 and the water temperature T_(sens)detected by the water temperature sensor 401 are fed back to determinethe cathode flow rate, an amount unadjustable by changing the targetpressure P_(target) and the target temperature T_(target) iscomplemented by the cathode flow rate.

By doing so, the target pressure is first decreased and the cathodepressure regulating valve 22 is opened when the target wet statedecreases. Subsequently, the target cooling water temperature isincreased and the three-way valve 42 is controlled. Finally, the targetflow rate is increased and the rotation speed of the compressor 21 isincreased. By doing so, an increase in the rotation speed of thecompressor 21 is suppressed as much as possible. Although powerconsumption increases to deteriorate fuel economy with an increase inthe rotation speed of the compressor, power consumption is suppressed toimprove fuel economy in the present embodiment since an increase in therotation speed of the compressor 21 is suppressed as much as possible.

FIG. 5 are charts showing a problem during execution of the abovecontrol logic when the target wet state increases.

Since an increase in the rotation speed of the compressor 21 issuppressed as much as possible by doing as described above when thetarget wet state decreases, power consumption is suppressed to improvefuel economy.

However, it was found by the present inventors that the wet state cannotbe controlled as targeted in the above manner when the target wet stateincreases. Specifically, since being set based on the stack temperature(minimum stack temperature T_(min)) and the cathode flow rate (minimumcathode flow rate at the time of setting the maximum wet state, thetarget pressure P_(target) is unlikely to vary when the target wet stateincreases.

Thus, when the target wet state increases at time t21, the targetcathode flow rate Q_(target) first starts decreasing as shown in FIG. 5.

If a complete adjustment is not possible only by changing the targetcathode flow rate Q_(target), the target pressure P_(target) and thetarget temperature T_(target) start varying at time t22. Temperature haspoor responsiveness and is less likely to vary than pressure.Conversely, pressure varies earlier than temperature and cannotcomplement temperature. Thus, temperature deviates from a target, withresult that the wet state cannot be controlled as targeted.

FIG. 6 is a block diagram showing functions relating to the wet statecontrol of the controller when the target wet state increases.

In FIG. 6, the target pressure calculation block B101 corresponds to thecomplementary control unit as claimed, the target temperaturecalculation block B102 corresponds to the water temperature control unitas claimed and the target flow rate calculation block B103 correspondsto the priority control unit as claimed.

As shown in FIG. 6, when the target wet state increases, a temperature(calculated value) calculated based on the stack temperature (minimumstack temperature T_(min)) at the time of setting the maximum wet stateand higher than the minimum stack temperature T_(min), but lower thanthe water temperature T_(sens) detected by the water temperature sensor401 is used.

This calculated value is specifically described.

In the present embodiment, attention is focused on a manipulation amountfor manipulating the cooling water temperature and a temperature to beinput to the target pressure calculation block B101 is calculatedaccording to this manipulation amount.

The manipulation amount for manipulating the cooling water temperatureis, for example, a rotation speed of the water pump 43.

With a decrease in the rotation speed of the water pump 43, thetemperature of the fuel cell stack 10 increases since the flow rate ofthe cooling water is small. If the temperature of the fuel cell stack 10increases, the amount of moisture contained in the cathode gas O₂increases. As a result, the moisture discharged together with thecathode gas O₂ also increases. Thus, the wet state of the electrolytemembrane decreases to dry the electrolyte membrane.

Conversely, the more it is attempted to dry the electrolyte membrane bydecreasing the wet state of the electrolyte membrane, the lower therotation speed of the water pump 43 becomes. The more it is attempted toincrease the wet state of the electrolyte membrane, the higher therotation speed of the water pump 43 becomes.

Accordingly, the lower the rotation speed of the water pump 43, the moreit is attempted to dry the electrolyte membrane by decreasing the wetstate of the electrolyte membrane. If the rotation speed of the waterpump 43 is minimum, it is attempted to drastically decrease the wetstate of the electrolyte membrane. Thus, at this time, the stacktemperature (minimum stack temperature T_(min)) at the time of settingthe maximum wet state is used as described as the above control when thetarget wet state decreases.

On the other hand, the higher the rotation speed of the water pump 43is, the more it is attempted to wet the electrolyte membrane byincreasing the wet state of the electrolyte membrane. If the rotationspeed of the water pump 43 is maximum, it is attempted to drasticallyincrease the wet state of the electrolyte membrane. Thus, at this time,a temperature (calculated value) calculated based on the stacktemperature (minimum stack temperature T_(min)) at the time of settingthe maximum wet state and higher than the minimum stack temperatureT_(min), but lower than the water temperature T_(sens) detected by thewater temperature sensor 401 is used. It should be noted that thistemperature constantly coincides with the water temperature T_(sens)detected by the target wet state 401.

During that time, the temperature is calculated based on the rotationspeed of the water pump 43. Specifically, a temperature T_(coolant) iscalculated in accordance with the following equation (7).

$\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 7} \right\rbrack & \; \\{T_{coolant} = {{\frac{T_{sens} - T_{\min}}{2} \times \frac{T_{pump\_ target}}{R_{pump\_ max}}} + T_{\min}}} & (7)\end{matrix}$where:

R_(pump) _(_) _(target): target rotation speed [rpm] of water pump 43

R_(pump) _(_) _(max): maximum rotation speed [rpm] of water pump 43

The temperature calculated in this way is shown in FIG. 7. Specifically,when it is attempted to dry the electrolyte membrane by decreasing thewet state of the electrolyte membrane, the stack temperature (minimumstack temperature T_(min)) at the time of setting the maximum wet stateis used. When it is attempted to wet the electrolyte membrane byincreasing the wet state of the electrolyte membrane, the temperature(calculated value) calculated based on the stack temperature (minimumstack temperature T_(min)) at the time of setting the maximum wet stateand higher than the minimum stack temperature T_(min), but lower thanthe water temperature T_(sens) detected by the water temperature sensor401 is used.

It should be noted that, as shown in FIG. 7, the temperature T_(coolant)is calculated by connecting the minimum stack temperature T_(min) andthe water temperature T_(sens) by a straight line and apportioningbetween the minimum stack temperature T_(min) and the water temperatureT_(sens) in the above Equation (7). However, there is no limitation tosuch a technique. The minimum stack temperature T_(min) and the watertemperature T_(sens) may be in a relationship connected by a downwardconvex curve such as an exponential function or may be in a relationshipconnected by an upward convex curve such as a log function instead of bya straight line. Such relationship should be set in advance. Thetemperature T_(coolant) may be calculated based on these curves.

FIG. 8 are timing charts showing the operation of the wet state controldevice when the target wet state increases.

By this way, a temperature lower than the water temperature T_(sens)detected by the water temperature sensor 401 is constantly input to thetarget pressure calculation block B101. Thus, the target wet statecannot be achieved at the pressure calculated in the target pressurecalculation block B101. Thus, the target wet state is achieved at thetemperature calculated by the target temperature calculation block B102and the pressure is varied to complement that temperature.

By this way, the target wet state of the fuel cell is changed, and thetarget flow rate first decreases and the rotation speed of thecompressor 21 decreases when the wet state is increased. Subsequently,the target cooling water temperature decreases and the three-way valve42 is controlled. Finally, the target pressure increases and the cathodepressure regulating valve 22 is closed. By doing so, the rotation speedof the compressor 21 is reduced as early as possible. As describedabove, power consumption increases to deteriorate fuel economy with anincrease in the rotation speed of the compressor. In other words, powerconsumption is suppressed to improve fuel economy with a decrease in therotation speed of the compressor. Since the rotation speed of thecompressor 21 is reduced as early as possible in the present embodiment,fuel economy is improved.

Further, since the temperature input to the target pressure calculationblock B101 is not suddenly switched depending on whether or not thetarget wet state decreases or increases, it can be avoided that thecontrol becomes unstable.

Second Embodiment

FIGS. 9A and 9B are block diagrams showing functions relating to a wetstate control of a controller of a second embodiment of a wet statecontrol device for fuel cell according to the present invention. FIG. 9Ashows functions when a target wet state increases and FIG. 9B showsfunctions when the target wet state decreases.

The wet state control device of the present embodiment includes a targetpressure calculation block B201, a target temperature calculation blockB202 and a target flow rate calculation block B203.

It should be noted that, when the target wet state increases (FIG. 9A),the target pressure calculation block B201 corresponds to thecomplementary control unit as claimed, the target temperaturecalculation block B202 corresponds to the water temperature control unitas claimed and the target flow rate calculation block B203 correspondsto the priority control unit as claimed.

When the target wet state decreases (FIG. 9B), the target pressurecalculation block B201 corresponds to the priority control unit asclaimed, the target temperature calculation block B202 corresponds tothe water temperature control unit as claimed, and the target flow ratecalculation block B203 corresponds to the complementary control unit asclaimed.

The target pressure calculation block B201 calculates a target cathodepressure P_(target) based on a target water discharge quantity Q_(H2O)_(_) _(out), a flow rate Q_(sens) detected by the cathode flow sensor201 and a water temperature T_(sens) detected by the water temperaturesensor 401. Specifically, the target cathode pressure P_(target) iscalculated by the following Equations (8-1), (8-2).

$\begin{matrix}\left\lbrack {{Equations}\mspace{14mu} 8} \right\rbrack & \; \\{{P_{target} = {P_{sat\_ sens}\frac{Q_{sens} - Q_{H\; 2{O\_ out}}}{Q_{H\; 2{O\_ out}}}}}{{where}\text{:}}} & \left( {8\text{-}1} \right) \\{P_{sat\_ sens} = 10^{7.7406 - \frac{1657.46}{227.02 + T_{sens}}}} & \left( {8\text{-}2} \right)\end{matrix}$

P_(sat) _(_) _(sens) denotes a saturated water vapor pressure at thewater temperature T_(sens) detected by the water temperature sensor 401and Equation (8-2) is obtained based on Antoine equation.

In the above manner, the target pressure calculation block B201calculates the target cathode pressure P_(target) based on the targetwater discharge quantity Q_(H2O) _(_) _(out), the actual flow rateQ_(sens) and the water temperature T_(sens).

The target temperature calculation block B202 calculates a targettemperature T_(target) based on the target water discharge quantityQ_(H2O) _(_) _(out), a minimum cathode pressure P_(min) and the flowrate Q_(sens) detected by the cathode flow rate sensor 201.Specifically, the target temperature T_(target) is calculated by thefollowing Equations (9-1), (9-2). It should be noted that Equation (9-1)is obtained by the reverse of Antoine equation.

$\begin{matrix}\left\lbrack {{Equations}\mspace{14mu} 9} \right\rbrack & \; \\{{T_{target} = {\frac{1657.46}{7.7406 - {\log_{10}P_{sat\_ target}}} - 227.02}}{{where}\text{:}}} & \left( {9\text{-}1} \right) \\{P_{sat\_ target} = {P_{\min}\frac{Q_{H\; 2{O\_ out}}}{Q_{sens} + Q_{H\; 2{O\_ out}}}}} & \left( {9\text{-}2} \right)\end{matrix}$

P_(sat) _(_) _(target) is a target saturated water vapor pressure.

In the above manner, the target temperature calculation block B202calculates the target temperature T_(target) based on the target waterdischarge quantity Q_(H2O) _(_) _(out), the minimum cathode pressureP_(min) and the flow rate Q_(sens) detected by the cathode flow ratesensor 201.

The target flow rate calculation block B203 calculates a target cathodeflow rate Q_(target) based on the target water discharge quantityQ_(H2O) _(_) _(out), a maximum stack temperature T_(max) and the minimumcathode pressure P_(min) as shown in FIG. 9A when the target waterdischarge quantity Q_(H2O) _(_) _(out) decreases, i.e. in the case ofwetting by increasing the wet state. Specifically, the target cathodeflow rate Q_(target) is calculated by the following Equations (10-1),(10-2).

$\begin{matrix}\left\lbrack {{Equations}\mspace{14mu} 10} \right\rbrack & \; \\{{Q_{target} = {Q_{H\; 2{O\_ out}}\frac{P_{\min} - P_{sat\_ max}}{P_{sat\_ max}}}}{{where}\text{:}}} & \left( {10\text{-}1} \right) \\{P_{sat\_ max} = 10^{7.7406 - \frac{1657.46}{227.02 + T_{\max}}}} & \left( {10\text{-}2} \right)\end{matrix}$

Here, the maximum stack temperature T_(max) is a stack temperature atthe time of setting the fuel cell stack in a minimum wet state. Asdescribed above, the temperature of the fuel cell stack 10 is increasedto decrease the wet state of the electrolyte membrane. It should benoted that a power generation failure may be caused by condensed waterif the temperature of the fuel cell stack 10 is too low. On the otherhand, if this temperature is too high, the deterioration of the fuelcell stack 10 is accelerated. Thus, the stack temperature at the time ofsetting the fuel cell stack in the minimum wet state is a highest stacktemperature in a range where the performance of the fuel cell stack canbe ensured, comprehensively considering these. Similarly, the minimumcathode pressure P_(min) is a cathode pressure at the time of settingthe fuel cell stack in the minimum wet state. As described above, thecathode pressure is decreased to decrease the wet state of theelectrolyte membrane. It should be noted that performance may bedeteriorated due to an insufficient pressure if the cathode pressure istoo low. On the other hand, if the cathode pressure is too high, it maynot be possible to realize by the compressor. Thus, the cathode pressureat the time of setting the fuel cell stack in the minimum wet state is alowest cathode pressure in a range where the performance of the fuelcell stack can be ensured, comprehensively considering these. These areset according to the operating state of the fuel cell by an experimentin advance.

Further, the target flow rate calculation block B203 calculates thetarget cathode flow rate Q_(target) based on a temperature (calculatedvalue) calculated based on the target water discharge quantity Q_(H2O)_(_) _(out) and the stack temperature (maximum stack temperatureT_(max)) and lower than the maximum stack temperature T_(max), buthigher than the water temperature T_(sens) detected by the watertemperature sensor 401 and the minimum cathode pressure P_(min) as shownin FIG. 9B when the target water discharge quantity Q_(H2O) _(_) _(out)increases, i.e. at the time of drying by decreasing the wet state. Thiscalculated value is obtained, considering the rotation speed of thewater pump 43 (manipulation amount for manipulating the cooling watertemperature) as in the first embodiment.

In the above manner, the target flow rate calculation block B203calculates the target cathode flow rate Q_(target).

FIG. 10 are timing charts showing the operation of the wet state controldevice when the target wet state increases.

When the above control logic is executed, the wet state control deviceoperates as follows when the target wet state increases.

When the target wet state increases at time t31, the wet state controldevice starts operating.

The target cathode flow rate Q_(target) is set based on the target waterdischarge quantity Q_(H2O) _(_) _(out), the maximum stack temperatureT_(max) and the minimum cathode pressure P_(min). The target temperatureT_(target) is set based on the target water discharge quantity Q_(H2O)_(_) _(out), the minimum cathode pressure P_(min) and the actual flowrate Q_(sens). The target cathode pressure P_(target) is set based onthe target water discharge quantity Q_(H2O) _(_) _(out), the actual flowrate Q_(sens) and the actual water temperature T_(sens).

Since being set based on the stack temperature (maximum stacktemperature T_(max)) and the cathode pressure (minimum cathode pressureP_(min)) at the time of setting the minimum wet state, the target flowrate Q_(target) is most likely to vary. Accordingly, the target flowrate Q_(target) is first preferentially decreased. Then, the compressor21 is controlled to realize this target flow rate Q_(target). Then, thecathode flow rate decreases almost without any response delay.

If a complete adjustment is not possible only by changing the targetflow rate Q_(target), the target temperature T_(target) starts varyingat time t32. Specifically, a limit value (minimum cathode pressureP_(min)) is used to set the target temperature T_(target). Further, thesensor detection value Q_(sens) of the cathode flow rate regulated asdescribed above is fed back. Thus, an amount unadjustable only by thecathode flow rate is adjusted by changing the temperature of the coolingwater. It should be noted that the temperature of the cooling water isunlikely to vary and a response delay is likely to occur even if atarget value is changed. Since the temperature of the cooling water isdetected by the water temperature sensor 401 and fed back to determinethe cathode pressure, a response delay of the cooling water temperatureis complemented by the cathode pressure.

If a complete adjustment is not possible even if the target temperatureT_(target) is changed, the target cathode pressure P_(target) startsvarying at time t33. Specifically, since the flow rate Q_(sens) detectedby the cathode flow rate sensor 201 and the water temperature T_(sens)detected by the water temperature sensor 401 are fed back to determinethe cathode pressure, an amount unadjustable by changing the target flowrate Q_(target) and the target temperature T_(target) is complemented bythe cathode pressure.

By this way, the target flow rate is first decreased and the rotationspeed of the compressor 21 is decreased when the target wet state of thefuel cell is changed to increase the wet state. Subsequently, the targetcooling water temperature is decreased and the three-way valve 42 iscontrolled. Finally, the target pressure is increased and the cathodepressure regulating valve 22 is closed. By doing so, the rotation speedof the compressor 21 is reduced as early as possible. As describedabove, power consumption increases to deteriorate fuel economy with anincrease in the rotation speed of the compressor. In other words, powerconsumption is suppressed to improve fuel economy with a decrease in therotation speed of the compressor. Since the rotation speed of thecompressor 21 is reduced as early as possible in the present embodiment,fuel economy is improved.

FIG. 11 are timing charts showing the operation of the wet state controldevice when the target wet state decreases.

Since an increase in the rotation speed of the compressor 21 issuppressed as much as possible in the above manner when the target wetstate increases, power consumption is suppressed to improve fueleconomy.

However, the wet state cannot be controlled as targeted in the abovemanner when the target wet state decreases. Specifically, since beingset based on the stack temperature (maximum stack temperature T_(max))and the cathode pressure (minimum cathode pressure P_(min)) at the timeof setting the minimum wet state, the target flow rate Q_(target) isunlikely to vary when the target wet state decreases.

If a complete adjustment is not possible only by changing the targetcathode pressure P_(target), the target flow rate Q_(target) and thetarget temperature T_(target) start varying at time t42. Temperature haspoor responsiveness and is less likely to vary than flow rate.Conversely, flow rate varies earlier than temperature and cannotcomplement temperature. Thus, temperature deviates from a target, withresult that the wet state cannot be controlled as targeted.

Contrary to this, in the present embodiment, the target flow ratecalculation block B203 uses the temperature (calculated value)calculated based on the stack temperature (maximum stack temperatureT_(max)) and lower than the maximum stack temperature T_(max), buthigher than the water temperature T_(sens) detected by the watertemperature sensor 401 when the target water discharge quantity Q_(H2O)_(_) _(out) increases, i.e. at the time of drying by decreasing the wetstate.

By this way, a temperature higher than the water temperature T_(sens)detected by the water temperature sensor 401 is constantly input to thetarget flow rate calculation block B203. Thus, the target wet statecannot be achieved at the flow rate calculated in the target flow ratecalculation block B203. Thus, the target wet state is achieved at thetemperature calculated by the target temperature calculation block B202and the flow rate is varied to complement that temperature.

By doing so, the target pressure is first decreased and the cathodepressure regulating valve 22 is opened when the target wet statedecreases. Subsequently, the target cooling water temperature isincreased and the three-way valve 42 is controlled. Finally, the targetflow rate is increased and the rotation speed of the compressor 21 isincreased. By doing so, an increase in the rotation speed of thecompressor 21 is suppressed as much as possible. Power consumptionincreases to deteriorate fuel economy with an increase in the rotationspeed of the compressor. However, since an increase in the rotationspeed of the compressor 21 is suppressed as much as possible in thepresent embodiment, power consumption is suppressed to improve fueleconomy.

Third Embodiment

FIGS. 12A and 12B are block diagrams showing functions relating to acontrol of a controller in a third embodiment of a wet state controldevice for fuel cell according to the present invention.

The wet state control device of the present embodiment includes a wetstate decreasing unit 100 and a wet state increasing unit 200.

The wet state decreasing unit 100 is a control unit executed when thetarget water discharge quantity Q_(H2O) _(_) _(out) increases, i.e. whenthe wet state decreases. The wet state decreasing unit 100 includes atarget pressure calculation block B101, a target temperature calculationblock B102 and a target flow rate calculation block B103. It should benoted that the wet state decreasing unit 100 is not described in detailsince being similar to a corresponding configuration of the firstembodiment (FIG. 3). It should be noted that the target pressurecalculation block B101 corresponds to the priority control unit asclaimed, the target temperature calculation block B102 corresponds tothe water temperature control unit as claimed, and the target flow ratecalculation block B103 corresponds to the complementary control unit asclaimed.

The wet state increasing unit 200 is a control unit executed when thetarget water discharge quantity Q_(H2O) _(_) _(out) decreases, i.e. whenthe wet state increases. The wet state increasing unit 200 includes atarget flow rate calculation block B203, a target temperaturecalculation block B202 and a target pressure calculation block B201. Itshould be noted that these blocks are not described in detail sincebeing similar to those of the second embodiment (FIG. 9A). It should benoted that the target pressure calculation block B201 corresponds to thecomplementary control unit as claimed, the target temperaturecalculation block B202 corresponds to the water temperature control unitas claimed, and the target flow rate calculation block B203 correspondsto the priority control unit as claimed.

FIG. 13 are timing charts showing the operation of the wet state controldevice when the target wet state decreases.

When the above control logic is executed, the wet state control deviceoperates as follows when the target wet state decreases.

When the target wet state decreases at time t11, the wet statedecreasing unit 100 of the wet state control device starts operating.

A target pressure P_(target) is set based on a target water dischargequantity Q_(H2O) _(_) _(out), a minimum stack temperature T_(min) and aminimum cathode flow rate Q_(min). A target temperature T_(target) isset based on the target water discharge quantity Q_(H2O) _(_) _(out), anactual pressure P_(sens) and the minimum cathode flow rate Q_(min). Atarget cathode flow rate Q_(target) is set based on the target waterdischarge quantity Q_(H2O) _(_) _(out), the actual pressure P_(sens) andan actual water temperature T_(sens).

Since being set based on the stack temperature (minimum stacktemperature T_(min)) and the cathode flow rate (minimum cathode flowrate Q_(min)) at the time of setting a maximum wet state, the targetpressure P_(target) is most likely to vary. Accordingly, the targetpressure P_(target) is first preferentially decreased. Then, the cathodepressure regulating valve 22 is controlled to realize this targetpressure P_(target). Then, the cathode pressure decreases almost withoutany response delay.

If a complete adjustment is not possible only by changing the targetpressure P_(target), the target temperature T_(target) starts varying attime t12. Specifically, a limit value (minimum cathode flow rate is usedto set the target temperature T_(target). Further, the sensor detectionvalue P_(sens) of the cathode pressure regulated as described above isfed back. Thus, an amount unadjustable only by the cathode pressure isadjusted by changing the temperature of the cooling water. It should benoted that the temperature of the cooling water is unlikely to vary anda response delay is likely to occur even if a target value is changed.Since the temperature of the cooling water is detected by the watertemperature sensor 401 and fed back to determine the cathode flow rate,a response delay of the cooling water temperature is complemented by thecathode flow rate.

If a complete adjustment is not possible even if the target temperatureT_(target) is changed, the target cathode flow rate Q_(target) startsvarying at time t13. Specifically, since the pressure P_(sens) detectedby the cathode pressure sensor 202 and the water temperature T_(sens)detected by the water temperature sensor 401 are fed back to determinethe cathode flow rate, an amount unadjustable by changing the targetpressure P_(target) and the target temperature T_(target) iscomplemented by the cathode flow rate.

FIG. 14 are timing charts showing the operation of the wet state controldevice when the target wet state increases.

When the target wet state increases at time t21, the wet stateincreasing unit 200 of the wet state control device starts operating.

The target cathode flow rate Q_(target) is set based on the target waterdischarge quantity Q_(H2O) _(_) _(out), the maximum stack temperatureT_(max) and the minimum cathode pressure P_(min). The target temperatureT_(target) is set based on the target water discharge quantity Q_(H2O)_(_) _(out), the minimum cathode pressure P_(min) and the actual flowrate Q_(sens). The target cathode pressure P_(target) is set based onthe target water discharge quantity Q_(H2O) _(_) _(out), the actual flowrate Q_(sens) and the actual water temperature T_(sens).

Since being set based on the stack temperature (maximum stacktemperature T_(max)) and the cathode pressure (minimum cathode pressureP_(min)) at the time of setting a minimum wet state, the target flowrate Q_(target) is most likely to vary. Accordingly, the target flowrate Q_(target) is first preferentially decreased. Then, the compressor21 is controlled to realize this target flow rate Q_(target). Then, thecathode flow rate decreases almost without any response delay.

If a complete adjustment is not possible only by changing the targetflow rate Q_(target), target temperature T_(target) starts varying attime t22. Specifically, a limit value (minimum cathode pressure P_(min))is used to set the target temperature T_(target). Further, the sensordetection value Q_(sens) of the cathode flow rate regulated as describedabove is fed back. Thus, an amount unadjustable only by the cathode flowrate is adjusted by changing the temperature of the cooling water. Itshould be noted that the temperature of the cooling water is unlikely tovary and a response delay is likely to occur even if a target value ischanged. Since the temperature of the cooling water is detected by thewater temperature sensor 401 and fed back to determine the cathodepressure, a response delay of the cooling water temperature iscomplemented by the cathode pressure.

If a complete adjustment is not possible even if the target temperatureT_(target) is changed, the target cathode pressure P_(target) startsvarying at time t23. Specifically, since the flow rate Q_(sens) detectedby the cathode flow rate sensor 201 and the water temperature T_(sens)detected by the water temperature sensor 401 are fed back to determinethe cathode pressure, an amount unadjustable by changing the target flowrate Q_(target) and the target temperature T_(target) is complemented bythe cathode pressure.

According to the present embodiment, the target pressure is firstdecreased and the cathode pressure regulating valve 22 is opened whenthe target wet state of the fuel cell is changed to decrease the wetstate. Subsequently, the target cooling water temperature is increasedand the three-way valve 42 is controlled. Finally, the target flow rateis increased and the rotation speed of the compressor 21 is increased.By doing so, an increase in the rotation speed of the compressor 21 issuppressed as much as possible. Power consumption increases todeteriorate fuel economy with an increase in the rotation speed of thecompressor. However, since an increase in the rotation speed of thecompressor 21 is suppressed as much as possible in the presentembodiment, power consumption is suppressed to improve fuel economy.

Further, according to the present embodiment, the target flow rate isfirst decreased and the rotation speed of the compressor 21 is decreasedwhen the target wet state of the fuel cell is changed to increase thewet state. Subsequently, the target cooling water temperature isdecreased and the three-way valve 42 is controlled. Finally, the targetpressure is increased and the cathode pressure regulating valve 22 isclosed. By doing so, the rotation speed of the compressor 21 is reducedas early as possible. As described above, power consumption increases todeteriorate fuel economy with an increase in the rotation speed of thecompressor. In other words, power consumption is suppressed to improvefuel economy with a decrease in the rotation speed of the compressor.Since the rotation speed of the compressor 21 is reduced as early aspossible in the present embodiment, fuel economy is improved.

Furthermore, in the present embodiment, the control logic of the targetpressure calculation block B101 and that of the target pressurecalculation block B201 are the same. Further, the control logic of thetarget temperature calculation block B102 and that of the targettemperature calculation block B202 are the same. Furthermore, thecontrol logic of the target flow rate calculation block B103 and that ofthe target flow rate calculation block B203 are the same. By changingonly signals to be input to these control blocks, the fuel cell is driedby decreasing the wet state of the fuel cell while being wetted byincreasing the wet state of the fuel cell. In this way, the wet state ofthe fuel cell can be controlled by changing a control priority of thepressure, the temperature and the flow rate only by changing inputvalues while having the same control logics.

Fourth Embodiment

FIGS. 15A and 15B are block diagrams showing functions relating to acontrol of a controller in a fourth embodiment of a wet state controldevice for fuel cell according to the present invention.

Depending on an operation mode, the three-way valve 42 is notcontrolled. Further, there is a possibility that the three-way valve 42cannot be controlled due to a certain trouble. At times like this, thetarget pressure calculation block B101 calculates the target pressureP_(target) using the water temperature T_(sens) detected by the watertemperature sensor 401 instead of the minimum stack temperature T_(min).Further, the target flow rate calculation block B203 calculates thetarget cathode flow rate Q_(target) using the water temperature T_(sens)detected by the water temperature sensor 401 instead of the maximumstack temperature T_(max).

By doing as in the present embodiment, an operation mode in which thethree-way valve 42 is not controlled and a case where the three-wayvalve 42 cannot be controlled due to a certain trouble can be dealtwith. Further, also in the present embodiment, the target pressure isfirst decreased and the cathode pressure regulating valve 22 is openedwhen the target wet state of the fuel cell is changed to decrease thewet state. Subsequently, the target flow rate is increased and therotation speed of the compressor 21 is increased. This causes anincrease in the rotation speed of the compressor 21 to be suppressed asmuch as possible, whereby power consumption is suppressed to improvefuel economy. Further, the target flow rate is first decreased and therotation speed of the compressor 21 is decreased when the target wetstate of the fuel cell is changed to increase the wet state.Subsequently, the target pressure is increased and the cathode pressureregulating valve 22 is closed. This causes the rotation speed of thecompressor 21 to be reduced as early as possible, whereby powerconsumption is suppressed to improve fuel economy.

Fifth Embodiment

FIGS. 16A and 16B are block diagrams showing functions relating to acontrol of a controller in a fifth embodiment of a wet state controldevice for fuel cell according to the present invention.

Depending on an operation mode, the cathode pressure regulating valve 22is not controlled. Further, there is a possibility that the cathodepressure regulating valve 22 cannot be controlled due to a certaintrouble. At times like this, the target flow rate calculation block B203calculates the target cathode flow rate Q_(target) using the pressuredetected by the cathode pressure sensor 202 instead of the minimumcathode pressure P_(min). Further, the target temperature calculationblock B202 calculates the target temperature T_(target) using thepressure P_(sens) detected by the cathode pressure sensor 202 instead ofthe minimum cathode pressure P_(min).

By doing as in the present embodiment, an operation mode in which thecathode pressure regulating valve 22 is not controlled and a case wherethe cathode pressure regulating valve 22 cannot be controlled due to acertain trouble can be dealt with. Further, also in the presentembodiment, the target cooling water temperature is first increased andthe three-way valve 42 is opened when the target wet state of the fuelcell is changed to decrease the wet state. Subsequently, the target flowrate is increased and the rotation speed of the compressor 21 isincreased. This causes an increase in the rotation speed of thecompressor 21 to be suppressed as much as possible, whereby fuel economyis improved. Further, the target flow rate is first decreased and therotation speed of the compressor 21 is decreased when the target wetstate of the fuel cell is changed to increase the wet state.Subsequently, the target cooling water temperature is decreased and thethree-way valve 42 is controlled. This causes the rotation speed of thecompressor 21 to be reduced as early as possible, whereby fuel economyis improved.

Sixth Embodiment

FIGS. 17A and 17B are block diagrams showing functions relating to acontrol of a controller in a sixth embodiment of a wet state controldevice for fuel cell according to the present invention.

Depending on an operation mode, the compressor 21 is not controlled.Further, there is a possibility that the compressor 21 cannot becontrolled due to a certain trouble. At times like this, the targetpressure calculation block B101 calculates the target cathode pressureP_(target) using the flow rate Q_(sens) detected by the cathode flowrate sensor 201 instead of the minimum cathode flow rate Q_(min).Further, the target temperature calculation block B102 calculates thetarget temperature T_(target) using the flow rate Q_(sens) detected bythe cathode flow rate sensor 201 instead of the minimum cathode flowrate Q_(min).

By doing as in the present embodiment, an operation mode in which thecompressor 21 is not controlled and a case where the compressor 21cannot be controlled due to a certain trouble can be dealt with.

Although the embodiments of the present invention have been described,the above embodiments are only an illustration of some applicationexamples of the present invention and the technical scope of the presentinvention is not limited to the specific configurations of the aboveembodiments.

For example, although the rotation speed of the water pump 43 isillustrated as the manipulation amount for manipulating the coolingwater temperature, there is no limitation to this. The manipulationamount may be an opening of the three-way valve 42 or the rotation speedof the cooling fan 410.

Further, also in the second embodiment, the temperature T_(coolant) maybe calculated, considering the manipulation amount for manipulating thecooling water temperature as in the first embodiment.

Furthermore, in addition to that, the above embodiments may beappropriately combined.

Further, the wet state of the fuel cell may be water balance of the fuelcell (for example, water balance is defined to be: “water balance=waterto be produced−water to be discharged”), may be resistance of theelectrolyte membrane of the fuel cell or may be another indicatorindicating the wet state of the fuel cell.

Furthermore, the temperature of the fuel cell itself or the temperatureof air may be used instead of the temperature of the cooling water.

Furthermore, in each of the above embodiments, the stack temperature(minimum stack temperature T_(min)) and the cathode flow rate (minimumcathode flow rate Q_(min)) at the time of setting the maximum wet stateare used in the target pressure calculation block B101 when the targetpressure P_(target) is set. The cathode flow rate (minimum cathode flowrate Q_(min)) at the time of setting the minimum wet state is used inthe target temperature calculation block B202 when the targettemperature T_(target) is set. The stack temperature (maximum stacktemperature T_(max)) and the cathode pressure (minimum cathode pressureP_(min)) at the time of setting the minimum wet state are used in thetarget flow rate calculation block B203 when the target flow rateQ_(target) is set. If the limit values (maximum values, minimum values)are used in this way, a largest effect is achieved. However, valuessmaller than the maximum values and those larger than the minimum valuesmay also be used. Even with such setting, a reasonable effect isachieved.

This application claims priorities of Japanese Patent Application No.2011-126109 filed with the Japan Patent Office on Jun. 6, 2011 andJapanese Patent Application No. 2011-165322 filed with the Japan PatentOffice on Jul. 28, 2011, all the contents of which are herebyincorporated by reference.

The invention claimed is:
 1. A wet state control device configured tocontrol a wet state of a membrane in a fuel cell, the wet state controldevice comprising: a priority control unit configured to control eitherone of a pressure or a flow rate of cathode gas when the wet state ofthe membrane is adjusted; a water temperature control unit configured tocontrol a temperature of cooling water when the wet state of themembrane is not completely adjustable by a control of the prioritycontrol unit; and a complementary control unit configured to control theother of the pressure or the flow rate of the cathode gas to complementa response delay of the water temperature control unit; wherein the fuelcell is dried by decreasing the wet state of the membrane, and duringthe drying: the priority control unit decreases the pressure of thecathode gas; the water temperature control unit increases thetemperature of the cooling water when the wet state of the membrane isnot completely adjustable by the control of the priority control unit;and the complementary control unit controls the flow rate of the cathodegas to complement the response delay of the water temperature controlunit.
 2. The wet state control device according to claim 1, wherein thefuel cell is dried by decreasing the wet state of the membrane, andduring the drying: the priority control unit decreases the pressure ofthe cathode gas based on a flow rate of the cathode gas that is to besupplied when the membrane is set in a wetter state than a present wetstate and the temperature of the cooling water when the membrane is setin a wetter state than the present wet state; the water temperaturecontrol unit increases the temperature of the cooling water based on anactual pressure of the cathode gas and the flow rate of the cathode gasthat is to be supplied when the wetter state than the present wet stateis set; and the complementary control unit controls the flow rate of thecathode gas based on the actual pressure of the cathode gas and anactual temperature of the cooling water.
 3. The wet state control deviceaccording to claim 2, wherein: the flow rate of the cathode gas that isto be supplied when the wetter state than the present wet state is setis a lowest flow rate in a range where the performance of the fuel cellis ensured; and the temperature of the cooling water that is to besupplied when the wetter state than the present wet state is set is alowest temperature in a range where the performance of the fuel cell isensured.
 4. The wet state control device according to claim 2, wherein:the priority control unit uses the actual temperature of the coolingwater instead of the temperature of the cooling water supplied when thewetter state than the present wet state is set if the water temperaturecontrol unit is not operating and the fuel cell is dried by decreasingthe wet state of the membrane.
 5. The wet state control device accordingto claim 2, wherein: the priority control unit and the water temperaturecontrol unit use an actual flow rate of the cathode gas instead of theflow rate of the cathode gas supplied when the wetter state than thepresent wet state is set if the complementary control unit is notoperating and the fuel cell is dried by decreasing the wet state of themembrane.
 6. The wet state control device according to claim 1, whereinthe fuel cell is dried by decreasing the wet state of the membrane, andduring the drying: the priority control unit decreases the pressure ofthe cathode gas based on an actual flow rate of the cathode gas and anactual temperature of the cooling water; the water temperature controlunit increases the temperature of the cooling water based on the actualflow rate of the cathode gas and a pressure of the cathode gas suppliedwhen the membrane is set in a drier state than a present wet state; andthe complementary control unit controls the flow rate of the cathode gasbased on a pressure of the cathode gas supplied when the membrane is setin the drier state than the present wet state and a temperature betweena temperature of the cooling water supplied when the membrane is set inthe drier state than the present wet state and an actual temperature ofthe cooling water.
 7. The wet state control device according to claim 6,wherein: the pressure of the cathode gas supplied when the drier statethan the present wet state is set is a lowest pressure in a range wherethe performance of the fuel cell is ensured; and the temperature of thecooling water supplied when the drier state than the present wet stateis set is a highest temperature in a range where the performance of thefuel cell is ensured.